Saying that u have a headache as an excuse to not do something, avoid doing something, to get someone to shut the fuck up or to get someone to do something.

Example 1
Man: Honey, we have been together for almost six months and i really wanna express my feelings for you physical..

Woman: Not now! I have a headache!

Example 2
Mother: My son went a party Friday night, he played soccer on Saturday and went to the cinema on Sunday but now he says that his head is hurting him and he cannot go to school tomorroe. What a convenient headache..